Method and arrangement for loop test of a disturbed line

ABSTRACT

The invention refers to single-ended test of a loop ( 2, 3 ). A test device (TD 1 ) is connected to a remote device ( 3 ) via a line ( 2 ). The remote device is powered on and transmits intermittently handshake signals (HS 1 ), which can disturb the loop test. A receiving and calculating unit (RE 1 ) detects the handshake signals (HS 1 ) and orders a sending unit (SD 1 ) to send a halting signal (NAK-EF) to the remote device. The latter is halted for predetermined silent period of time, during which the test or a part of it is performed. If required the halting of the handshake signals is repeated. The test device sends a broadband loop test signal (S 1 ) and receives a reflected signal (S 2 ) during the silent period. A frequency dependent echo transfer function is generated from the signals (S 1 , S 2 ) and is used for generating desired properties of the line ( 2 ) such as its length (L).

CROSS-REFERENCE TO RELATED APPLICATION

This application is a non-provisional under 35 U.S.C. 119(e) of U.S. provisional application No. 60/469,658 filed on May 12, 2003 and is also a continuation under 35 U.S.C. 120 of PCT International Application number PCT/SE2004/000566 filed on Apr. 08, 2004, the disclosures of which is incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and an arrangement in the area of single-ended test of a signal line being subjected to disturbances from a remote end device.

DESCRIPTION OF RELATED ART

In today's telecommunication it is essential from an economical point of view to use existing copper wires for broadband transmission. These copper wires, often called twisted-pair copper loops or copper access lines, have among themselves very different properties from a broadband point of view. Telecom operators therefore have a great interest in testing the properties of the lines to be able to fully utilize their transmission capacity. The abovementioned is discussed in an article by Walter Goralski: “xDSL Loop Qualification and Testing”, IEEE Communications Magazine, May 1999, pages 79-83. The article also discusses testing possibilities and test equipment.

The transmission properties of copper lines are more closely discussed in an article by José E. Schutt-Ainé: “High-Frequency Characterization of Twisted-Pair Cables”, IEEE Transactions on Communications, Vol. 49, No. 4, April 2001. Propagation parameters of high bit rate digital subscriber twisted-pair cables are extracted by a wave propagation method model. The frequency dependence in the properties of the transmission line and the influence of the skin effect on these are studied.

Testing the transmission properties of a line can be performed by sending a test signal from one end of the line and measure it at the other end, so called double end test. That method is labour intensive and expensive. A more frequently used method is to send a test signal from one end of the line and measure on the reflected pulse, so called Single-Ended Loop Testing, SELT. In an article by Stefano Galli and David L Waring: “Loop Makeup Identification Via Single Ended Testing: Beyond Mere Loop Qualification”, IEEE Journal on Selected Areas in Communications, Vol. 20, No. 5, June 2002, is discussed the influence of different types of line discontinuities and generated echoes in connection with single-ended testing. A mathematical method for handling the echoes is presented and also an experimental validation of the method.

It is an obvious choice from a technical point of view to use a laboratory-type measurement device for performing a SELT. Using such a device is however expensive. Irrespective of that, the measurement can be influenced by disturbances which arises when a Customer Premises Equipment (CPE), that is connected to the remote end of the line, is trying to perform a handshake procedure. The handshake procedure makes it difficult to analyse the measured echo-frequency response and the normal noise on the line.

In single-ended testing it is advantageous to, instead of the laboratory device, use a transceiver as a part of a measurement device for the loop under test. The broadband communication transceiver is however no perfect voltage generator but introduces distortion in the measurement. How to remove this distortion is discussed in a standardization paper by Thierry Pollet:“How is G.selt to specify S₁₁ (calibrated measurements)?”, ITU Telecommuni-cation Standardization Sector, Temporary Document OJ-091; Osaka, Japan 21-25 October, 2002. A calibration method is presented, based on a one port scattering parameter S₁₁, that includes transceiver parameters which are generated during a calibration. Also in a standardization paper by Thierry Pollet: “Minimal information to be passed between measurement and interpretation unit”, ITU Telecommunication Standardization Sector, Temporary Document OC-049; Ottawa, Canada 5-9 August, 2002, the one port scattering parameter S₁₁ is discussed. Also when using the transceiver for the SELT the remote CPE can disturb the measurement by trying to perform a handshake procedure.

SUMMARY OF THE INVENTION

The present invention is concerned with the abovementioned problem how to avoid the influence of a handshake procedure on a single-ended loop test of a copper access line connected to a CPE. As long as the line stays inactivated the powered on CPE will try to perform a handshake procedure transmitting intermittent handshake signals. Due to these handshake signals it is difficult to analyse a measured echo-frequency response when the connected CPE modem is powered on.

Another problem arises when a transceiver is utilized in the single-ended test of the line. The problem is how to also compensate for the influence on the SELT measurement of the transceiver itself.

Still a problem is how to generate and store transceiver values for the compensation.

The problem is solved in the following manner. In the handshake procedure the CPE transmits intermittent narrowband signals, handshake tones, of predetermined frequencies. The handshake tones are detected by the device performing the SELT measurement and the handshake tones are halted for a time interval. During this interval the SELT measurement is performed, if necessary after repeated halts of the handshake tones.

When using the transceiver for the SELT measurement, the problems in connection with that are solved by calibrating a test transceiver, which is a typical broadband communication transceiver, and generate transceiver model values. These values are stored and are used in the transceiver for communication purposes, which is connected to the loop to be tested. A test signal, as reflected by the loop, is measured at the communication transceiver, giving a loop test result. The influence on this result by the communication transceiver itself is compensated for with the aid of the stored transceiver model values.

A purpose with the present invention is to improve the SELT measurement of the access line, when the CPE sends its intermittent handshake signals.

Another purpose with the present invention is to compensate for the influence of a transceiver on the SELT testing of the line.

Still a purpose is to generate and store transceiver values for the compensation.

An advantage with the present invention is that the SELT measurement of the access line can be performed when the CPE sends its intermittent handshake lines.

Another advantage with the invention is that the influence of the transceiver on the SELT measurement of a copper access line can be compensated for.

A further advantage is that transceiver values for the compensation can be generated and stored and can be applied for all standard broadband transceivers, based on the same hardware as the tested one. Hence a costly procedure of calibrating an actual transceiver will be eliminated.

Still an advantage is that the generated transceiver values have an easily understandable meaning.

Still another advantage is that the test transceiver can be any one of the transceivers used for communication purposes.

The invention will now be more closely described with the aid of embodiments and with reference to the enclosed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simple block schematic over a test device and a transmission line;

FIG. 2 shows a frequency diagram with handshake tones;

FIG. 3 shows a time diagram with periodic handshake sequences;

FIG. 4 shows a flow chart for SELT measurement;

FIG. 5 shows a simple block schematic over a transceiver and the line;

FIG. 6 shows a somewhat more detailed block schematic over a part of the transceiver and the line;

FIG. 7 shows a block schematic over the transceiver connected to an impedance of known value;

FIG. 8 shows a flow chart for generating of transceiver characteristic values; and

FIG. 9 shows a flow chart for generating of an impedance value for the line.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a simple block schematic over a test device TD1 at a central office, connected to a remote device 3 at a customer's premises via a digital subscriber line 2 (DSL). The line is seen from the end of the test device, and this end is called the near end of the line while the other end at the device 3 is denoted as the remote end. The line 2 is a conventional copper line of a length L, which has certain properties, such as signal attenuation in different frequency ranges. The test device has a sending device SD1, a receiving device RE1 and a synchronizing device SH1. The latter is connected to the sending device SD1, which in turn is connected to the line 2 and to the receiving device RE1.

As mentioned above it is essential for a network operator to be able to utilize the already existing copper line 2 for the broadband transmission. The operator therefore must know the line properties, such as the length L, signal attenuation and transmission capacity. These properties can normally be determined after a measurement, which is advantageously performed from the near end of the line as a so called Single-Ended Loop Test, SELT. The parameters are related to a line input impedance Z_(in)(ƒ) which can be evaluated using transmitted and reflected test signals.

The test device TD1 transmits such a test signal, a broadband loop test signal S1, which is reflected by the remote device 3 and is measured by the test device as a reflected signal S2. With the aid of a quotient S2/S1 parameters of the line 2 can be determined, as will be described in detail below. The measurement of the reflected signal S2 can however be disturbed by the remote device 3, which tries to perform a handshake procedure. To avoid this disturbance, first the presence of a handshake signal must be detected.

During the handshake procedure the remote device 3 intermittently sends a narrowband handshake signal HS1, which can interfere with the signal S2 and render the line measurement more difficult. The interfering handshake signal can however be detected. The loop test signal S1, used in the present embodiment, has a frequency in the range of 0-1104 kHz or higher. It is based on a measurement signal that has a number of orthogonal frequency points coinciding with the so called Discrete Multi Tone signal. This signal is used for modulation purposes in the ADSL standard ITU-T G.992.1 The test device, that measures the reflected signal S2 in this frequency range 0-1104 kHz, therefore can be used for the detection of the handshake signal. A noise signal in the range of 0-276 kHz or higher is also to be measured by the test device and also this measurement can be used for detecting the handshake signal HS1. The handshake tones in standardized DSL transmission due to the ITU-T standard G.994.1 uses narrowband signals modulated with plain DPSK scheme. Due to its limited bandwidth it is fairly simple to distinguish the individual frequency position of these handshake signals. In for example ADSL there are three sets of mandatory upstream handshake tones, based on a frequency f₀=4.3125 kHz:

-   ADSL Annex A : N=[9, 17, 25] -   ADSL Annex B : N=[37, 45, 53] -   ADSL Annex C : N=[7, 9]

The handshake tones have the frequency F=N×f₀. One or more of the above tones may be transmitted during handshake for one specific annex setting.

The above handshake signals can be observed as narrowband disturbance in a noise measurement, delivering a mean value noise floor over frequency. In FIG. 2 is shown handshake tone disturbance in such a noise measurement. The figure is a diagram with the frequency ƒ in kHz on the abscissa and the signal level A in dBm/Hz on the ordinate. In a frequency range from about 200 kHz and higher the noise NS1 has a rather constant level. In the range 0-200 kHz the handshake signal HS1 can be observed. The figure shows that the particular remote device 3 connected has four tones active during the handshake. By identifying these frequency tones in the spectra it appears that it is an ADSL Annex A set, fulfilling the mandatory set of handshake tones, and with an optional additional tone from Annex B.

Further, handshake tones are transmitted following a standardized time scheme. The signals are not continuous but sent with periodic intervals leaving the line silent in between. For example the standard ITU-T G994.1 states:

“If a NAK-EF message is received in any state, the receiving station shall return immediately to the initial G.994.1 state (R-SILENT0 for an HSTU-R, C-SILENT1 for an HSTU-C) and remain silent for a minimum period of 0.5 s. It may then initiate another G.994.1 session.”

This means that if an operator wants to perform a SELT measurement of the line 2 from the test device TD1, the handshake procedure from the remote device 3 can be halted. The handshake signals HS1, e.g. as described in FIG. 2, are indicated by the receiving device RE1. The synchronizing device SH1 orders the sending device SD1 to send the NAK-EF message, which halts the handshake tones HS1 for a time interval of at least 0.5 seconds. During this interval the loop test signal S1 is sent from the sending device on order from the synchronizing device SH1 and the reflected signal S2 is received in the receiving device RF1. To get full information about the line 2 it may be necessary to resend the loop test signal S1 and repeat the SELT measurement procedure. The complete measurement will then follow FIG. 3, which is a diagram with the time T on the abscissa, showing SELT measurement synchronized with handshake signals. The figure shows the handshake signals HS1 followed by the NAK-EF message and interleaving time intervals TI1, used for the SELT measurement. It should be noted that it is only the SELT measurement that is to be performed in the time intervals TI1. The calculations concerning the line properties based on the SELT measurement can go on continuously.

As an alternative to the standard ITU-T G.994.1 there is an ADSL standard ANSI T1E1.413 using a different method for handshake.

The SELT measurement method described above will be summarized in connection with a flow chart in FIG. 4. In a step 401 the customer premises equipment CPE, the remote end device 3, is connected to the remote end of the line 2. The test device is connected to the near end of the line in a step 402 and in a step 403 the handshake signals HS1 are transmitted from the CPE. In a step 404 the handshake signals HS1 are indicated in the receiving device RE1. The handshake signals are halted for the time interval TI1 of predetermined duration in a step 405. In a next step 406 the SELT measurement is performed and in a step 407 it is investigated whether the SELT measurement is ready. In an alternative NO1 the method step 404 is repeated with an indication of the handshake signals. Then the method steps 405, 406 and 407 are repeated until the alternative YES1 after the step 407 is the actual one and the procedure ends in a step 408.

Below will be described in an embodiment how the single-ended loop test, the SELT, is performed.

In FIG. 5 is shown a transceiver 1 connected to the remote device 3 via the line 2. The transceiver is suitable for communication purposes and is described such that the SELT measurement can be explained. The transceiver 1 includes a digital part 41, a codec 42 and an analog part 43, the so called Analog Front End AFE. The digital part includes in turn a digital signal generator 13 and a computational device 11 interconnected with a memory device 12. The transceiver 1 also has an input 63 and an output 64. The generator, which is connected to the computational device 11, sends a broadband input loop test signal v_(in) to the remote device 3 via the codec 42, the analog part 43 and the line 2. A reflected broadband loop test signal v_(out) is received in the computational device from the line 2 via the analog part and the codec.

The broadband loop test signal v_(in), sent for such measuring purposes, is reflected back over the line 2 and is noted as the loop test signal v_(out). As will be described below, the signals v_(in), and v_(out) are used in the determining of the properties of the line 2.

What the operator in fact needs to know is the input impedance Z_(in)(ƒ) of the line 2 including the remote device 3, measured from a transceiver interface 5 and being independent of the transceiver 1 itself. A first step in getting the required line properties is to generate an echo transfer function H_(echo)(ƒ) for the actual line 2. This is calculated by performing a frequency translation of the broadband signals v_(in) and v_(out), resulting in signals V_(in)(ƒ) and V_(out)(ƒ) in the frequency domain. The transfer function is generated by the relationship H _(echo)(ƒ)=V _(out)(ƒ)/V _(in)(ƒ)   (1) in which the frequency is denoted by ƒ.

Naturally, the function H_(echo)(ƒ) includes properties of the transceiver 1. Below it will be described by an example how the required line properties of the line 2 can be obtained with the aid of the frequency dependent echo transfer function H_(echo)(ƒ). First, the transceiver analog part 43 will be described somewhat more in detail in connection with FIG. 6. This is to throw light upon the difficulties in characterizing the transceiver 1 in a simple manner.

FIG. 6 is a simplified block diagram over the analog transceiver part 43 and the line 2 of FIG. 5, yet somewhat more detailed than in that figure. The analog part 43 includes an amplifier block 6, a hybrid block 7, a sense resistor RS and a line transformer 8. The amplifier block 6 has a driver 61 with its input connected to the digital generator 13 via the codec 42, not shown. I also has a receiver 62 receiving signals from the line 2 and having its output connected to the transceiver digital part 41, not shown. The driver output is connected to the sense resistor RS, the terminals of which are connected to the hybrid block 7. The latter has four resistors R1, R2, R3 and R4 and is connected to inputs of the receiver 62. The line transformer 8 has a primary winding L1 and two secondary windings L2 and L3 interconnected by a capacitor C1. The primary winding L1 is connected to the sense resistor RS and the secondary windings L2 and L3 are connected to the line 2. The frequency dependent line input impedance at the interface 5 is denoted Z_(in)(ƒ) and the input impedance at the primary side of the transformer is denoted ZL. The termination of the far-end of the line 2, the remote device 3, is represented by an impedance ZA.

The signal v_(in), now in analog form from the codec 42, is amplified in the driver block 61. The output impedance of the driver is synthezised by the feedback loop from the sense resistor RS. The line transformer 8 has a voltage step-up from the driver to the loop. The capacitor C1 has a DC-blocking function. The transformer and the capacitor act as a high pass filter between the driver 61/receiver 62 and the loop 2, 3 with a cut-off frequency around 30 kHz. No galvanic access to the loop is possible in this case.

In the present description a frequency-domain model of the echo transfer function H_(echo)(ƒ) is used to calculate the frequency dependent input impedance Z_(in)(ƒ) of the loop 2 and 3, as seen by the transceiver 1 at the interface 5. The input impedance can then be used for calculating several loop qualification parameters. This frequency-domain model of the echo transfer function H_(echo)(ƒ) includes three parameters Z_(h0)(ƒ), Z_(hyb)(ƒ) and H_(∞)(ƒ) which relate to the transceiver 1. The parameters, transceiver model values, fully describe the transceiver from this point of view.

The parameters Z_(h0)(ƒ), Z_(hyb)(ƒ) and H_(∞)(ƒ) are originally deduced analytically from the circuits of the transceiver. Some minor simplifications have been made in the analysis, but the model has proved to be very accurate. In the enclosed Appendix 1, “Simulation of the echo transfer function for DAFE708” it is shown how the model of the echo transfer function H_(echo)(ƒ) is derived.

The values of the parameters are normally not calculated directly from the component values of the transceiver, but are generated from measurements in a calibration process, as will be described below.

In the earlier mentioned standardization paper “How is G.selt to specify S₁₁ (calibrated measurements)?” the scattering parameter S₁₁ is expressed with three parameters C1, C2 and C3 for the transceiver. These parameters should not be confused with the transceiver model values Z_(h0)(ƒ), Z_(hyb)(ƒ) and H_(∞)(ƒ) of the present description. The parameters C1, C2 and C3 are dimensionless quantities and are not given any concrete meaning, although they are successfully used to model the transceiver. The transceiver model values of the present description are recognized in the analysis and can be interpreted directly:

The value H_(∞)(ƒ) is the frequency dependent echo transfer function for the transceiver 1 with open connection to the line 2, i.e. when the line impedance is of unlimited magnitude.

The value Z_(hyb)(ƒ) is the transceiver impedance as measured at the connections to the line 2, i.e. the transceiver impedance at the interface 5 as seen from the line side.

The value Z_(h0)(ƒ) can be expressed as Z_(h0)(ƒ)=H₀(ƒ)·Z_(hyb)(ƒ), in which the value H₀(ƒ) is the frequency dependent echo transfer function for the transceiver 1 with the connections to the line 2 shortcut and the value Z_(hyb)(ƒ) is defined above.

It is to observe that the transceiver model values are not measured directly, but are generated in a process as will be described below.

The echo transfer function H_(echo)(ƒ) of equation (1) can be expressed as:

$\begin{matrix} {{H_{echo}(f)} = \frac{{{H_{\infty}(f)}{Z_{in}(f)}} + {Z_{h\; 0}(f)}}{{Z_{in}(f)} + {Z_{hyb}(f)}}} & (2) \end{matrix}$ in which

Z_(in)(ƒ) is the earlier mentioned input impedance of the line 2 as a function of the frequency ƒ; and

Z_(h0)(ƒ), Z_(hyb)(ƒ) and H_(∞)(ƒ) are complex vectors and are the transceiver model values mentioned above.

After a calibration measurement of a certain transceiver version its vectors can be determined. These vectors, the transceiver model values, are then pre-stored in for example the software of the transceivers of the measured version, e.g. in the memory 12 of the transceiver 1. The model values are then used for the loop test of the line 2 with its initially unknown properties.

In connection with FIG. 7 will be mentioned how the calibration measurement is performed. The figure shows a test transceiver 31, to which test impedances 9 of different predetermined values are connected at the interface 5 for the line 2. A measurement device 32 with a memory 33 is connected to the input 63 and the output 64 of the test transceiver. The measurement device 32 sends a control signal VC1 to the test transceiver 31 and initiates it to generate a broadband transceiver test signal vt_(in), one for each value of the test impedance 9. A reflected output transceiver test signal vt_(out) is received in the test transceiver, which sends a corresponding control signal VC2 to the measurement device. A complete measurement requires the measurement of three selected impedance values. The echo transfer function H_(echo)(ƒ) is then generated in accordance with the relationship (1).

Using three impedance values for the calibration is sufficient to generate the transceiver values. To get more precise values, more than the three impedances can be used. This gives rise to an overdetermined equation system. An example on a set of standard values of the test impedance 9 for the calibration is an open circuit, a shortcut circuit and an impedance value corresponding to an expected value for the loop, e.g. 100 ohms. It should be noted that a value for a purely resistive component is normally valid only up to a limited frequency, e.g. 1 MHz. For higher frequencies it is recommended to measure the impedance value of the “resistive” component.

The generating of the three complex vectors Z_(h0)(ƒ), Z_(hyb)(ƒ) and H_(∞)(ƒ) for the measured transceiver 31 is performed in the following manner. The model of the echo transfer function in the relationship (2) can be expressed as:

$\begin{matrix} {{\left( {1 - {{H_{echo}(f)}{Z_{in}(f)}}} \right)\begin{pmatrix} {Z_{ho}(f)} \\ {Z_{hyb}(f)} \\ {H_{\infty}(f)} \end{pmatrix}} = {{H_{echo}(f)}{Z_{in}(f)}}} & (3) \end{matrix}$ or equivalently Ax=b, where

${A = \left( {1 - {{H_{echo}(f)}{Z_{in}(f)}}} \right)},\mspace{11mu}{x = {{\begin{pmatrix} {Z_{ho}(f)} \\ {Z_{hyb}(f)} \\ {H_{\infty}(f)} \end{pmatrix}\mspace{14mu}{and}\mspace{14mu} b} = {{H_{echo}(f)}{Z_{in}(f)}}}}$

The general solution to the system Ax=b is x=(A ^(T) A)⁻ A ^(T) b

By using the values of the transfer function H_(echo)(ƒ), measured as described above with different types of the input terminations 9, the vector x can be solved. The thus generated calibration values of the vector x are stored for example in the memory 33 of the measurement device 32 or in the memory 12 of the transceivers of the measured version. Note that A, x and b normally are complex valued and frequency dependent.

After a measurement of the echo transfer function H_(echo)(ƒ) for the actual unknown line 2, its input impedance as seen by the transceiver 1 at the interface 5 can be generated as:

$\begin{matrix} {{Z_{in}(f)} = \frac{{Z_{h\; 0}(f)} - {{Z_{hyb}(f)}{H_{echo}(f)}}}{{H_{echo}(f)} - {H_{\infty}(f)}}} & (4) \end{matrix}$

To summarize, a certain hardware for transceivers like the transceiver 1 is first calibrated. This is performed for the test transceiver 31 with the aid of the impedances 9 and the transceiver test signals vt_(in) and vt_(out). The vector x is calculated and the values of the vector x are stored and can be used for any transceiver with the same hardware. The echo transfer function H_(echo)(ƒ) is then measured by the transceiver 1 for the line 2 having unknown properties with the aid of the loop test signals v_(in) and v_(out). The frequency dependent input impedance Z_(in)(ƒ) of the line 2, as seen from the transceiver interface 5, is then generated.

In the embodiment described above, both the transceiver test signals vt_(in), vt_(out) and the loop test signals v_(in),v_(out) have been broadband signals. It is possible to use signals of any desired frequency width both for the calibration and the measurement of the line. The calibration and the loop test will of course be valid only for the selected frequency range. It has been mentioned that the transceiver model values are stored in the memory 12 of the transceiver 1. An obvious alternative is to store the values in the memory 33 or in a memory in some central computer and transmit them to the transceiver 1 when they are required for the generating of e.g. the input impedance Z_(in)(ƒ) of the line 2. Also, in the description has been mentioned the test transceiver 31 and the transceiver 1 for communication purposes. The test transceiver 31 can be any of a set of transceivers which are based on one and the same hardware. The test transceiver can in an obvious way be used for the communication purposes.

The above generating of transceiver model values and the generating of the impedance value for the line 2 will be shortly described in connection with flowcharts in FIGS. 8 and 9.

In FIG. 8 is shown the generating and storing of the transceiver model values. The method begins in a step 601 with the selection of the transceiver 31 for test purposes. In a step 602 an impedance 9 with a predetermined value is selected and in a step 603 the impedance is connected to the line connection of the test transceiver 31. In a step 604 the transceiver test signal vt_(in) is sent through the transceiver 31 to the line 2. To get transceiver model values that can be used for a wide range of applications the test signal is a broadband signal. The signal is reflected by the remote device 3 and after passage of the transceiver 31 it is received as the transceiver test signal vt_(out) in a step 605. In a step 606 the echo transfer function H_(echo)(ƒ) is generated in the computational device 32 for the actual impedance 9, after first having transformed the signals vt_(in) and vt_(out) into the frequency domain. In a step 607 it is investigated whether measurements for a sufficient number of the impedances 9 have been made, so that the transceiver model values Z_(h0)(ƒ), Z_(hyb)(ƒ) and H_(∞)(ƒ) can be generated. In an alternative NO1 a further impedance 9 is selected in the step 602. For an alternative YES1 the transceiver model values Z_(h0)(ƒ), Z_(hyb)(ƒ) and H_(∞)(ƒ) are generated in a step 608. In a step 609 the vector x, i.e. the transceiver model values, are stored in the memory 33. Next, the transceiver 1 for communication purposes is selected in a step 610. In a step 611 the transceiver model values Z_(h0)(ƒ), Z_(hyb)(ƒ) and H_(∞)(ƒ) are transmitted to the selected transceiver 1 and are stored in the memory 12.

FIG. 9 shows the generating of the frequency dependent line input impedance Z_(in)(ƒ) at the transceiver interface 5 to the line 2. In a step 701 the transceiver 1 for communication purposes is connected to the line 2 with the remote device 3. The loop test signal v_(in) is sent in a step 702. The loop test signal v_(out) as reflected by the line 2 is received by the transceiver and is measured in a step 703. In a step 704 the frequency dependent echo transfer function H_(echo)(ƒ) is generated in the computational device 11. The frequency dependent impedance value Z_(in)(ƒ) for the line 2 is generated in the device 11 with the aid of the stored transceiver model values and the echo transfer function, step 705. This generating is performed in accordance with the relationship (4). 

1. A method in a single-ended loop test, SELT, of a signal line, the method including: connecting a communication equipment to a remote end of the signal line; connecting a test device to a near end of the signal line, wherein the communication equipment transmits intermittent handshake signals on the signal line from the communication equipment to the test device; detecting the handshake signals in the test device; sending a halting message from the test device to the communication equipment to halt the handshake signals for a first time interval of predetermined duration; performing the SELT measurement during the first time interval, said performing step including: sending a loop test signal on the signal line from the test device to the communication equipment; and receiving at the test device, a reflected loop test signal; and determining properties of the signal line based on characteristics of the loop test signal and the reflected loop test signal.
 2. The method according to claim 1, wherein the SELT measurement is not completed during the first time interval, and the method further comprises: sending a second halting message from the test device to the communication equipment to halt the handshake signals for a second time interval; and completing the SELT measurement in the second time interval.
 3. The method according to claim 1, wherein the test device is a transceiver for communication purposes.
 4. The method according to claim 3, wherein a calibration process is performed for the transceiver for communication purposes, the method including: selecting a transceiver having the same type of hardware as said transceiver for communication purposes and including the transceiver for communication purposes; connecting at least three impedances each of a predetermined value to a line connection of the selected transceiver; generating for the selected transceiver frequency dependent echo transfer functions (H_(echo)(ƒ)) utilizing said at least three impedances and test signals(vt_(in), vt_(out)); and generating transceiver model values (Z_(h0)(ƒ), Z_(hyb)(ƒ), H_(∞)(ƒ)) with the aid of said echo transfer functions (H_(echo)(ƒ)) and the corresponding impedance values, said model values including: an echo transfer function (H_(∞)(ƒ)) for the test transceiver with open line connection, a transceiver impedance value (Z_(h0)(ƒ)) as seen from the line side, and a product (Z_(h0)(ƒ)) of said transceiver impedance value (Z_(hyb)(ƒ)) and an echo transfer function (H₀(ƒ)) for the transceiver with shortcut line connection.
 5. The method according to claim 4 including storing the transceiver model values (Z_(h0)(ƒ), Z_(hyb)(ƒ), H_(∞)(ƒ)) for performing the calibration process.
 6. The method according to claim 4 including storing the transceiver model values (Z_(h0)(ƒ), Z_(hyb)(ƒ), H_(∞)(ƒ)) in said transceiver for communication purposes.
 7. A system for performing a single-ended loop test, SELT, of a signal line, the system comprising: a test device having connections for a near end of the signal line; a receiving device in the test device for detecting intermittent handshake signals received on the signal line from a communication equipment at a remote end of the signal line, and for receiving a reflected loop test signal; a sending device in the test device for transmitting a halting message to the communication equipment, and for transmitting the loop test signal to the communication equipment, wherein the halting message halts the handshake signals from the communication equipment for a first time interval of predetermined duration; and means for determining properties of the signal line based on characteristics of the loop test signal and the reflected loop test signal; wherein the sending device transmits the loop test signal to the signal line and the receiving device receives the reflected loop test signal from the signal line during the first time interval.
 8. The system according to claim 7, wherein the sending device in the test device transmits a second halting message to the communication equipment to halt the handshake signals for a second time interval if the SELT measurement is not completed during the first time interval.
 9. The system according to claim 7, wherein the test device is a transceiver suitable for communication purposes.
 10. The system according to claim 9, wherein in a calibration mode, the system also comprises: a measurement device for generating, in the calibration process, calibration values for the transceiver with the aid of at least three impedances and test signals (vt_(in),vt_(out)), the impedances having each a predetermined value and being connected to the line connection of the transceiver; the measurement device being arranged to generate a frequency dependent echo transfer function (H_(echo)(ƒ)) for the test transceiver; and the measurement device being arranged to generate transceiver model values (Z_(h0)(ƒ), Z_(hyb)(ƒ), H_(∞)(ƒ)) with the aid of said echo transfer function (H_(echo)(ƒ)) and the corresponding impedance values, said model values including: an echo transfer function (H_(∞)(ƒ)) for the transceiver with open line connection, a transceiver impedance value (Z_(hyb)(ƒ)) as seen from the line side, and a product Z_(h0)(ƒ) of said transceiver impedance value (Z_(hyb)(ƒ)) and an echo transfer function (H₀(ƒ)) for the transceiver with shortcut line connection.
 11. The system according to claim 10, further including a memory for storing the transceiver model values (Z_(h0)(ƒ), Z_(hyb)(ƒ), H_(∞)(ƒ)). 